The American Quarter Horse is a breed of horse originally bred for sprinting short distances, typically races of a quarter mile or less. The Quarter Horse is the most popular breed in the United States, and is the largest breed society in the world, with over 3.2 million Quarter Horses registered worldwide.  It is commonly believed to be the world's fastest horse over short distances, and has been clocked at 63 mph.  The Quarter Horse is well known both as a race horse and for its performance in rodeos, horse shows and as a working ranch horse. The compact body of the Quarter Horse is well-suited to the intricate and speedy maneuvers required in calf roping, reining, cutting, working cow horse, barrel racing and other western riding events, especially those involving live cattle. The versatile Quarter Horse is also shown in English disciplines, driving, and many other equestrian activities.

Development of the Quarter Horse as a distinct breed

Early foundation sires of Quarter horse type included Steel Dust, foaled 1843; Shiloh (or Old Shiloh), foaled 1844; Old Cold Dock (1862); Lock's Rondo, one of many "Rondo" horses, foaled in 1880; Old Billy -- again, one of many "Billy" horses -- foaled circa 1880; Traveler, shipped to Texas in 1889; Lock's Rondo (1880); and the Thoroughbred stallion Peter McCue, foaled 1895. 

The main duty of the ranch horse in the American West was working cattle. Even after the invention of the automobile, horses were still irreplacable for handling livestock on the range. Thus, major Texas cattle ranches, such as the King Ranch, the 6666 (Four Sixes) Ranch, and the Waggoner Ranch played a significant role in the development of the modern American Quarter Horse.

The skills needed by ranch hands and their horses became the foundation of the rodeo, a contest which began with informal competition between cowboys and expanded to become a major competitive event throughout the west. To this day, the Quarter Horse dominates the sport both in speed events and in competition that emphasizes the handling of live cattle.

However, sprint races were also popular weekend entertainment and racing became a source of economic gain for breeders as well. As a result, more Thoroughbred blood was added back into the developing Quarter horse breed. The Quarter Horse also benefitted from the addition of Arabian, Morgan and even Standardbred bloodlines.

In 1940, the American Quarter Horse Association (AQHA) was formed by a group of horsemen and ranchers from the southwestern United States dedicated to preserving the pedigrees of their ranch horses. The horse honored with the first registration number, P-1, was Wimpy, a descendant of the King Ranch foundation sire Old Sorrel. Other foundation sires alive at the founding of the AQHA and given the earliest registration numbers included King, Peppy, Leo, Joe Reed, Poco Bueno, Joe Hancock, and Three Bars (a Thoroughbred). Other Thoroughbred sires seen in early Quarter Horse pedigrees include King Plaudit, Blob, Top Deck, Vandy and Truckle Feature.

Since the American Quarter Horse formally established itself as a breed, the AQHA stud book has remained open to Thoroughbreds. Quarter Horse/Thoroughbred crosses are entered into the registry as "Appendix Quarter Horses." These animals are popular for Quarter Horse racing and for jumping and hunter events. After meeting a series of conformational and performance criteria, these Appendix Quarter Horses can obtain permanent registration numbers. Since American Quarter Horse/Thoroughbred crosses continue to have an opportunity to enter the official registry of the American Quarter Horse breed, this is creating a continual gene flow from the Thoroughbred breed into the American Quarter Horse breed, which has been influential in altering many of the characteristics that typified the breed in the early years of its formation.

Breed Characteristics

The modern American Quarter Horse has a small, short, refined head with a straight profile, and a strong, well-muscled body, featuring a broad chest and powerful hindquarters. They usually stand 14-16 hands high, although some may grow as tall as 17 hands.

There are two main body types: the stock type and the racing type. The stock horse type is shorter, more compact, stocky and well muscled, yet agile. The racing Quarter Horse is built to sprint short distances ranging from 220 - 870 yards, and therefore is somewhat taller and smoother muscled than the stock type, more closely resembling the Thoroughbred.

Quarter Horses shown in-hand in "halter" (conformation) competition are larger horses, with a muscular appearance, small heads with wide jowls, and refined muzzles. Reining and cutting horses are smaller, with quick, agile movement and very powerful hindquarters. Western pleasure show horses are often slightly taller, with a relatively level topline and smooth gaits. Quarter Horse race horses have long legs and are much leaner than their stock horse counterparts. The show hunter type is similar to the running type Quarter Horse, although some are taller, slimmer and have an even more Thoroughbred-like appearance. However, all Quarter Horses have speed, stamina, power and a willingness to please.

Quarter Horses come in nearly all colors. The most prominent color is sorrel (a brownish red, sometimes called chestnut). Other recognized colors are bay, black, brown, buckskin, dun, red dun, gray, grullo, palomino, red roan, blue roan, bay roan, perlino, and cremello. In the past, spotted or pinto colors were excluded, but now with the advent of DNA testing to verify parentage, the registry accepts all colors and prints as long as parents are registered.

Genetic Diseases

There are genetic diseases of concern to Quarter Horse breeders:

• Hyperkalemic periodic paralysis (HYPP), which is caused by an autosomal dominant gene linked to the stallion Impressive. It is characterized by uncontrollable muscle twitching and substantial muscle weakness or paralysis among affected horses. Because it is a dominant gene, only one parent has to have the gene for it to be transmitted to offspring. There is a DNA test for HYPP, the AQHA requires testing and is now limiting registration of some horses who possess the gene.
• Hereditary Equine Regional Dermal Asthenia (HERDA), also known as hyperelastosis cutis (HC). This is caused by a recessive gene, and thus, unlike HYPP, HERDA can only be transmitted if both parents carry the gene. When a horse has this disease, there is a collagen defect that results in the layers of skin not being held firmly together. Thus, when the horse is ridden under saddle or suffers trauma to the skin, the outer layer often splits or separates from the deeper layer, or it can tear off completely. It rarely heals without disfiguring scars. Sunburn can also be a concern. In dramatic cases, the skin can split along the back and even roll down the sides, with the horse literally being skinned alive. Most horses with HERDA are euthanized for humane reasons between the age of two and four years. The very hotly debated and controversial theory, put forth by researchers at Cornell University and Mississippi State University is that the sire line of the great foundation stallion Poco Bueno is implicated as the origin of the disease. There currently is no DNA test for HERDA, but active research is ongoing to try to pinpoint the gene. In 2006, researchers at the UC Davis School of Veterinary Medicine were able to localize a gene that may be responsible for HERDA, though a test has yet to be developed. 
• Glycogen Branching Enzyme Deficiency (GBED) is a genetic disease where the horse is lacking an enzyme necessary for storing glycogen, the horse's heart muscle and skeletal muscles cannot function, leading to rapid death. The disease occurs in foals who are homozygous for the lethal GBED allele, meaning both parents carry one copy of the gene. There is a DNA blood test for this gene.

• "Crop-out" Quarter Horses with Paint markings were not allowed to be registered for many years, it is also believed that some Quarter Horses may also carry the gene for Lethal White Syndrome. There is also a DNA test for this condition.

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